Page 146 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

110 part I The Energy–atmosphere System
Urban heat island
Commercial
Urban
Downtown
Rural Suburban residential Park Rural farmland residential River farmland
°C
33 32 31 30
°F 92
90
85
▲Figure 4.17 Typical urban heat island profile. On average, urban temperatures may be 1–3 C° warmer than nearby rural areas on a sunny summer day. Temperatures are highest at the urban core. note the cooling over the park, river, and rural areas.
[Based on “Heat island,” Urban Climatology and Air Quality, available at weather.msfc.nasa.gov/urban/urban_heat_island.html.]
TABlE 4.1 Urban Heat Islands: Driving Factors and Climatic Response
Driving Factor
Climatic Element Affected
Urban Compared to rural
Explanation
Thermal properties of urban surfaces: metal, glass, asphalt, concrete, brick
Urban canyon effect
net radiation Higher
Wind speed less
• annual mean
• extreme gusts
Calm periods More
air pollution More
• condensation nuclei • particulates Cloudiness, including
fog
Precipitation More Snowfall, inner city More Snowfall, downwind less
More
•
•
• •
•
•
Urban surfaces conduct more energy than natural surfaces such as soil.
Reflected insolation in canyons is conducted into surface materials, thus increasing temperatures.
Buildings interrupt wind flows, diminishing heat loss through advection (horizontal movement), and block nighttime radiation to space.
Maximum UHi effects occur on calm, clear days and nights.
airborne pollutants (dust, particulates, aerosols) in urban dust dome raise temperatures by absorbing inso- lation and reradiating heat to surface.
increased particulates are condensation nuclei for water vapour, increasing cloud formation and precipitation; heating enhances convection processes.
Reflective properties of urban surfaces
albedo
lower
• Urban surfaces often have low albedo, so they absorb and retain heat, leading to high net radiation values.
anthropogenic heating
Temperature
• annual mean
• winter minima
• summer maxima
Higher
• Heat is generated by homes, vehicles, and factories.
• Heat output may surge in winter with power for heating
or in summer with power for air conditioning.
Pollution
Urban desert effect:
less plant cover and more sealed surfaces
Relative humidity • annual mean infiltration Runoff Evaporation
less
less More less
• Cooling effect of evaporation and plant transpiration is reduced or absent.
• Water cannot infiltrate through sealed surfaces to soil; more water flows as runoff.
• Urban surfaces respond as desert landscapes—storms may cause “flash floods.”
Late Afternoon Temperature